1. Field of the Invention
The present invention relates to a method of producing an oriented electrical steel sheet having superior magnetic properties, and more particularly, relates to a method of producing a grain oriented electrical steel sheet having a Goss crystal orientation expressed by the Miller Index as a {110}&lt;001&gt; orientation in which the {110} plane is parallel to the surface of a steel sheet and the &lt;100&gt; axis coincides with the rolling direction, or a double oriented electrical steel sheet having a Goss crystal orientation expressed by the Miller Index as a {100}&lt;001&gt; orientation.
These steel sheets having a superior magnetic property can be used as the core of a transformer and a generator, etc.
2. Description of the Related Art
The oriented electrical steel sheet is formed, as explained above, of a required oriented crystal grain and having a sheet thickness of 0.10 to 0.35 mm, and usually containing 4.5% or less of Si.
The oriented electrical steel sheet requires a good excitation property and a watt loss property as the magnetic properties thereof, and to obtain an oriented electrical steel sheet having superior magnetic properties, the orientation of the crystal grain must be precisely aligned. A high densification of the crystal orientation can be realized by using a grain growth phenomenon known as secondary recrystallization.
To control the secondary recrystallization, a control of a primary recrystallization structure before the secondary recrystallization and a control of a fine precipitate, called an inhibitor or grain segregation type element, are indispensable. The inhibitor prevents the growth of a general primary recrystallized grain in a primary recrystallized structure and causes a selective growth of crystal grains having a special orientation.
As a typical precipitate, M. F. Littmann (Japanese Examined Patent Publication (Kokoku) No. 30-3651) and J. E. May, D. Turnbull (Trans. Met. Soc. AIME 212 (1958) p. 769-781) propose MnS, Taguchi and Itakura (Japanese Examined Patent Publication (Kokoku) No. 40-15644) propose AlN, and Imanaka et al (Japanese Examined Patent Publication (Kokoku) No. 51-13469 MnSe, and Komatsu et al, propose (Al, Si)N respectively.
On the other hand, as grain boundary segregation type elements, Saito propose Pb, Sb, Nb, Ag, Te, Se, S, etc., in the Japanese Metal Society Journal 27 (1963) P 186-195, but these elements are merely used as an auxiliary of the precipitate type inhibitor in the industrial process.
Although the conditions necessary to realize the functions of the inhibitor are not clear, taking into account the results of Matsuoka ("Iron and Steel" 53 (1967) p 1007-1023) and Kuroki et al (Japanese Metal Society Journal 43 (1979) p. 175-181 and 44 (1980) p. 419-424 the conditions appear to be as follows.
(1) Before the secondary recrystallization an amount of fine precipitates sufficient to prevent the growth of the primary recrystallized grain exists. PA1 (2) The size of the precipitates is large to a certain degree, and it is not thermally rapidly changed in the secondary recrystallization annealing process.
Three methods of producing a typical grain oriented electrical steel sheet are well known, as follows.
The first method is carried out by a two stage cold-rolling process using MnS as an inhibitor, and this method is disclosed in the Japanese Examined Patent Publication (Kokoku) No. 30-3651 by M. F. Littmann. The second method is carried out by a process comprising a finishing cold rolling at a reduction ratio of 80% or more using AlN+MnS as an inhibitor, and is disclosed in Japanese Examined Patent Publication (Kokoku) No. 40-15644 by Taguchi and Sakakura. The third method is carried out by a two stage cold rolling process using MnS (or MnSe) + Sb as an inhibitor, and is disclosed in the Japanese Examined Patent Publication (Kokoku) No. 51-13469 by Imanaka.
In these production techniques, to a complete solid-dissolving of the inhibitor by heating at a high temperature of approximately 1400.degree. C. before the hot-rolling of slabs is a basic requirement for obtaining a sufficient amount of precipitates, and a miniaturization thereof. Nevertheless, the following problems arise when heating slabs at a high temperature.
(1) a high temperature slab heating furnace for only the oriented electrical steel sheet is needed.
(2) The energy consumption of the heating furnace is high and expensive.
(3) The oxidation of the slab surface is advanced, a melt called a slag is generated, the maintenance time for the heating furnace is increased, with the result that the maintenance costs become high and the furnace operating ratio is lowered.
To realize a low temperature slab heating overcoming the above problems, an inhibitor formation technique in which the high temperature slab heating is not used is required.
Some of the present invention proposed a method of producing an oriented electrical steel sheet wherein an inhibitor is formed by nitriding a steel sheet having a finishing thickness. A grain oriented electrical steel sheet and a double oriented electrical steel sheet are disclosed in Japanese Examined Patent Publication (Kokoku) No. 62-45285 and Japanese Unexamined Patent Publication (Kokai) No. 1-139722 respectively.
In these techniques, it is important that the inhibitor be uniformly precipitated in the surface of the steel sheet by a nitriding, but when the steel sheets are produced on an industrial scale, if the nitriding is nonuniformly effected in a length direction of strip and a width direction thereof, the magnetic properties of the products become nonuniform.
The rate-determining step of the nitriding is a reaction in the surface of the strip (steel sheet), and thus to obtain a uniform and stable nitriding it is important to control the oxidized layer formed on the surface in the primary recrystallization annealing.
The oxidized layer form a forsterite film in the finishing annealing process, by a chemical reaction with MgO coated on the surface of the steel sheet as the annealing separator. The forsterite film functions such that, when the products are used as a transformer in a stacked state. An isolation between the steel sheet is ensured, a tension can be provided thereto, and the watt loss property can be improved.